Based on the threshold switching characteristics in VO2 memristors, oscillatory neuronal behavior can be implemented with a simple circuit, whose configuration is depicted in Figure 3a. The VO2 volatile memristor is connected in series with a load resistor (RL), and the intrinsic capacitance of the VO2 memristor provides the dynamics for integration. Figure 3d shows the oscillation characteristics of the spiking neuron. When a suitable voltage (Vin) is applied and a matched serial resistor (RL) is connected, the voltage dividing effect between the RL and VO2 memristor results in a voltage drop over VO2 memristor that exceeds its Vth. Therefore, the VO2 device will switch from off to on state. Once the device is in on state, the voltage drops across RL and the VO2 device will be re-distributed, and the voltage gets lower than Vhold and hence the device returns to off state. To achieve such oscillation effect, the following requirements should be satisfied:
  \(\frac{V_{in}\cdot R_{off}}{R_{off}+R_L}\ge V_{th}\)         (1)
 \(\frac{V_{in}\cdot R_{on}}{R_{on}+R_L}\le V_{hold}\)        (2)
          When Equations (1) and (2) are satisfied, the above spike event will be produced and repeated (Figure 3d). Figure 3d and 3e show the oscillating behavior of the spiking neuron, when the RL or input voltage is varied, respectively. As expected, the oscillation frequency decreases (0.9, 0.7, 0.55, 0.35 MHz) as RL increases (3.0, 3.6, 4.2, 4.8 kΩ) (Figure 3d), whereas the rate increases (0.45, 0.65, 0.8, 0.9 MHz) as the input voltage increases (4.2, 4.8, 5.4, 6.0 V) (Figure 3e). The dependence of the oscillation frequency on the load resistor (RL) is systematically tested under different input voltages (4, 5, 6 V), and the results are summarized in Figure 3b, further demonstrating that the oscillation frequency decreases as RL increases in each Vin case. Similarly, Figure 3c systematically analyzes the dependence of the oscillation frequency on Vin, when varied RL (3.0, 3.5, 4.0, 4.5 kΩ) is adopted, implying the same increasing trend of oscillation frequency with increase of Vin. Therefore, the oscillation frequency of the spiking neuron can be effectively modulated by the threshold voltage of the VO2 memristor and circuit parameters (RL and Vin), which paves the way for the construction of an artificial sensory system.